The present invention relates to a gas sensing element incorporated in an air-fuel ratio sensor used for controlling an air-fuel ratio of the gas mixture introduced into a combustion chamber of an internal combustion engine.
In an automotive vehicle, an air-fuel ratio sensor is generally provided in an exhaust pipe of an internal combustion engine to control the air-fuel ratio of the gas mixture introduced into a combustion chamber of the engine.
According to voltage-current characteristics of an air-fuel ratio sensing element incorporated in this air-fuel ratio sensor, as later explained with reference to FIG. 7, a current value increases in proportion to an applied voltage in a region less than a first voltage value. Then, the current value does not vary after the applied voltage reaches and exceeds the first voltage value. This region is referred to as a flat region. And, after the voltage value reaches and exceeds a second voltage value, the current value again increases in proportion to the applied voltage.
The current value in the flat region is referred to as a limit current value. By utilizing the limit current value, the air-fuel ratio sensing element measures an air-fuel ratio of the internal combustion engine.
When numerous gas sensing elements of identical specification are manufactured, there will be the possibility that the limit current value of each sensing element may disperse due to manufacturing errors. However, to assure accurate measurement of the air-fuel ratio, each of the manufactured air-fuel ratio sensing elements need to produce an identical limit current value when exposed to the same measured gas.
In view of the above, U.S. Pat. No. 5,685,964 (corresponding to JP 8-193974) discloses a method of adjusting a limit current value by cutting or removing partly a diffusion resistive portion which is provided on a measured gas sensing electrode.
However, the above-described conventional method is complicated and inefficient in that precise machining processing needs to be applied to each manufactured sensing element. Another problem is that precisely performing the machining processing is very difficult.
In view of the above-described problems of the prior art, the present invention has an object to provide an output characteristics adjusting method for a gas sensing element which is capable of easily and precisely adjusting the output characteristics of each manufactured gas sensing element.
To accomplish the above and other related objects, the present invention provides a first method for adjusting output characteristics of a gas sensing element comprising a measured gas sensing electrode and a reference gas sensing electrode provided on surfaces of a solid electrolytic body. The first adjusting method of the present invention is characterized by the step of supplying electric power to the gas sensing element so as to adjust an output characteristic value to a target value.
The output characteristics adjustment according to the first adjusting method of the present invention is very simple in that the required operation is only supplying electric power to each manufactured gas sensing element. In other words, the first adjusting method of the present invention requires no modification (e.g., cutting or machining operation) to the gas sensing element itself. The electric power supply operation is performed for all of the manufactured gas sensing elements. The electric power supply operation is easily done without increasing costs. As a result, the output characteristics adjustment can be simply performed for all of the manufactured gas sensing elements.
The present invention provides a second method for adjusting output characteristics of a gas sensing element comprising the step of manufacturing a gas sensing element having an output characteristic value whose initial value is in a range offset from a target value, and a step of supplying electric power to the gas sensing element until the output characteristic value is adjusted from the initial value to the target value.
For example, a limit current value of the gas sensing element can be adjusted as the output characteristics according to the second adjusting method of the present invention. The limit current varies when electric power is supplied to the gas sensing element as later explained with reference to FIG. 5.
Considering this tendency or phenomenon, each gas sensing element is manufactured to have a limit current value whose initial value is slightly offset from (i.e., lower or higher than) a target value. Then, the manufactured gas sensing element is subjected to the output characteristics adjustment based on supply of electric power to the gas sensing element.
When electric power is applied, as later explained with reference to FIG. 4, the limit current value of each manufactured gas sensing element changes with elapsed time from the initial value (i.e., in a range M) to the target value (I0).
Supply of electric power is stopped when the limit current value reaches the target value (I0). Accordingly, the second adjusting method can be performed after each gas sensing element is manufactured without adding any modification to the gas sensing element and accordingly realizes efficient mass production of brand-new gas sensing elements having an accurate limit current value regardless of dispersion of their initial limit current values caused due to manufacturing errors or the like.
The output characteristics adjustment according to the second adjusting method is very simple in that a required operation is only supplying electric power to each manufactured gas sensing element. In other words, the second adjusting method of the present invention requires no modification to the gas sensing element itself. The electric power supply operation is performed for all of the manufactured gas sensing elements. The electric power supply operation is easily done without increasing costs. As a result, the output characteristics adjustment can be simply performed for all of the manufactured gas sensing elements.
According to the first and second adjusting methods of the present invention, it is preferable that the step of supplying electric power to the gas sensing element is performed in a lean atmosphere because an absolute value of the output characteristics, such as a limit current value, becomes large in a lean atmosphere. The output characteristics adjustment is easily and accurately done.
In this case, a atmosphere is referred to as an atmosphere which contains substantially no vaporized fuel and exhaust gas. In this respect, the air is the most preferable lean atmosphere for the electric power supply operation. Inactive gas, such as nitrogen gas and argon gas, can be also preferably used as an atmosphere for the electric power supply operation of this invention.
According to the first and second adjusting methods, it is also preferable that the step of supplying electric power to the gas sensing element is performed at an element temperature equal to or larger than an active temperature. The output characteristic value, such as a limit current value, varies depending on the temperature and is stabilized after the element temperature reaches the active temperature. Thus, the output characteristics adjustment is accurately performed by supplying electric power to the gas sensing element after the element temperature reaches the active temperature.
When the solid electrolytic body is made of zirconia, it is preferable that the step of supplying electric power to the gas sensing element is performed at an element temperature equal to or larger than 600xc2x0 C.
When the gas sensing element comprises at least one additional electrode other than the measured gas sensing electrode and the reference gas sensing electrode so as to constitute a plurality of cells, it is preferable that the step of supplying electric power to the gas sensing element is performed by using at least one of the plurality of cells.
Furthermore, it is preferable that the step of supplying electric power to the gas sensing element is performed in a limit current region.
The present invention provides a third method for adjusting a limit current value of a gas sensing element comprising a measured gas sensing electrode and a reference gas sensing electrode provided on surfaces of a solid electrolytic body. The third adjusting method comprises a step of manufacturing the gas sensing element so as to have a limit current value whose initial value is offset from a target value, and a step of supplying electric power to the manufactured gas sensing element until the limit current value is adjusted from the initial value to the target value.
According to the third adjusting method, it is preferable that the step of supplying electric power is performed by connecting a power supply circuit between the measured gas sensing electrode and the reference gas sensing electrode at an element temperature equal to or larger than an active temperature.
The present invention provides a fourth method for adjusting a limit current value of a gas sensing element comprising a reference gas sensing electrode and a measured gas sensing electrode provided on inner and outer surfaces of a cup-shaped solid electrolytic body. The fourth adjusting method comprises a step of manufacturing the gas sensing element so as to have a limit current value whose initial value is offset from a target value, a step of incorporating the manufactured gas sensing element into a sensor body with lead wires extending out of the sensor body from the reference gas sensing electrode and the measured gas sensing electrode respectively, a step of connecting a power supply circuit between the lead wires, a step of increasing the temperature of the gas sensing element to an active temperature with an electric heater placed in the cup-shaped solid electrolytic body, and a step of applying a voltage of the power supply circuit between the reference gas sensing electrode and the measured gas sensing electrode through the lead wires until the limit current value is adjusted from the initial value to the target value.
The present invention provides a fifth method for adjusting a limit current value of a multilayered gas sensing element comprising a first cell having a pair of electrodes formed on surfaces of a solid electrolytic sheet and a second cell having a pair of electrodes formed on surfaces of a solid electrolytic sheet. The fifth adjusting method comprises a step of manufacturing the gas sensing element so as to have a limit current value whose initial value is offset from a target value, a step of connecting a power supply circuit between lead terminals of at least one of the first and second cells, a step of increasing the temperature of the gas sensing element to an active temperature with an electric heater integrally formed with the solid electrolytic sheets so as to constitute a multilayered body, and a step of applying a predetermined voltage of the power supply circuit between the electrodes of at least one of the first and second cells until the limit current value is adjusted from the initial value to the target value.
According to the third to fifth adjusting methods, it is preferable that the gas sensing element is kept in a lean atmosphere during adjustment of the limit current value at a temperature equal to or larger than an active temperature.
The adjusting method of the present invention can be applied to various types of gas sensing elements used for detecting the concentration of specific gases, such as NOx, HC, and CO, based on the limit current value.
The electric power supply operation can be performed by using direct current or alternating current.